Energy storage system and method

ABSTRACT

A method and system are described for using compressed air for storing and utilizing energy. Air is compressed and stored and the heat of compression is removed from the air prior to storage. The air is subsequently removed from storage and the stored heat is restored to the air prior to expansion of the air for deriving work.

United States Patent Koutz [is] 3,677,008 [451 July 18,1972

I54! ENERGY STORAGE SYSTEM METHOD [72] Inventor: Stanley L. Koutz, SanDiego, Calif.

[73] Assignee: Gulf Oil Corporation, Pittsburgh, Pa.

[22] Filed: Feb. 12, 1971 [21] Appl. No.: 115,017

[52] U.S. Cl ..60/59 T [51] Int. Cl ..F0lk 27/00 [58] Field ofSearch..60/59 T, 59 R, 57 T, 56, 55, 60/26 [56] References Cited UNITED STATESPATENTS 1,231,051 6/1917 Nordberg ..60/59 R UX coolan! sourca 1,297,3633/1919 Kneedler ..60/59 R 2,268,074 12/1941 Keller... .....60/59 T2,62l,48l 12/1952 Bowden ..60/59 T Primary Examiner-Edgar W. GeogheganAttorney-Anderson, Luedeka, Fitch, Even and Tabin ABSTRACT A methodandsystem are described for using compressed air for storing and utilizingenergy. Air is compressed and stored and the heat of compression isremoved from the air prior to storage. The air is subsequently removedfrom storage and the stored heat is restored to the air prior toexpansion of the air for deriving work.

11 Claims, 2 Drawing Figures ENERGY STORAGE SYSTEM AND METHOD Thisinvention relates to systems for the storage and use of energy and, moreparticularly, to a method and system of using compressed air for storingand utilizing energy.

Because the demand for electrical power may vary considerably from timeto time, many public utilities have been searching for a means ofstoring large amounts of energy. By doing so, excess energy availableduring periods of low demand can be produced and stored until suchenergy is needed during periods of high demand. In this way, main powerplant facilities can be designed to produce electrical energy on arelatively constant basis, reducing their complexity and costs.

One means of providing for large-scale energy storage is to utilizeexcess electrical energy during off-peak periods to pump water to ahigher level where it is stored in a suitable reservoir. The storedpotential energy in the water is subsequently utilized to drivehydroelectric facilities during peak power need periods so thatadditional generators can be cut into the line. Such systems have thedisadvantage that they require suitable geographical features,substantial surface area, and are difficult and expensive to build.

An alternative means of energy storage which ofiers significantadvantages over the hydroelectric pumped storage, discussed above, isthe utilization of compressed air for energy storage. Typically such asystem involves the storage of compressed air in an excavated or naturalcavern which is hydrostatically pressurized from a suitable waterreservoir or is otherwise pressurized. Excess electrical power is usedto operate compressors which compress the air for storage during periodsof low electrical demand. The stored compressed air is then removedduring periods of high demand to drive turbines for operating additionalelectrical generation equipment during periods of high demand. Suchsystems have been described in articles in the published periodicjournals Business Week, Aug. 1, 1970, page 74; and MechanicalEngineering, Nov. 1970, page 20.

Known compressed air storage systems, although providing significantpromise for the purpose intended, typically require that essentially allof the heat of compression be discharged to air or cooling water inorder to increase the mount of air stored in a given cavity volume. Whenthe air is utilized for power production, it must be reheated byaddition of energy, e.g. by burning fossil fuel. The heat thus requiredinvolves additional energy input which may reduce the overall efficiencyand economy of the power plant.

Accordingly, it is an object of the present invention to provide animproved method and system of using compressed air for storing andutilizing energy.

Another object of the invention is to provide a method and system forusing compressed air for storing and utilizing energy in whichefficiency is substantially improved.

It is another object of the invention to provide a method and system ofusing compressed air for storing and utilizing energy in which theamount of fuel which must be burned is substantially reduced oreliminated.

Other objects of the invention will become apparent to those skilled inthe art from the following description, taken in connection with theaccompanying drawings wherein:

FIG. 1 is a schematic diagram illustrating a system constructed inaccordance with the invention for employing the method of the invention;and

FIG. 2 is a partial schematic diagram illustrating a modification of thesystem of FIG. 1.

Very generally, the method and system of the invention involve thecompression of a quantity of air and the storing of same in a suitablereservoir. A substantial amount of the heat of compression of the air isremoved prior to storage of the air and such removed heat is alsostored. Upon demand, at least part of the stored compressed air isremoved and a substantial amount of the stored heat of compression isrestored to heat the removed compressed air. Work is thereafter derivedfrom the heated compressed air by allowing it to expand.

The method of the invention may best be understood by reference to thedrawings in which a system constructed in accordance with the inventionis illustrated. In FIG. 1, ground level is indicated at 11. The belowground components of the system include a storage chamber or reservoir12 which is partially filled with water 13 to maintain a suitablehydrostatic pressure through an aqueduct 14 communicating with a surfacereservoir 16.

The mechanical aspects of the system of the invention which arepositioned above ground include an altemator/motor 17 which is connectedby a clutch 18 to a compressor 19. The compressor 19 is illustrated as asingle stage compressor in the drawings, but it is to be understood thatthe compressor 19 may be a series of compressors or compressor stages,as is known in the art. The clutch 18 couples the compressor drive shaft21 to the alternator/motor drive shaft 22.

The alternator drive shaft is also coupled through a clutch 23 to thedrive shaft 24 of a prime moving means or turbine 26. As was the case inconnection with the compressor 19, the turbine 26 is illustrated as asingle stage, but it is to be understood that the turbine could comprisea plurality of turbines or turbine stages.

During periods of low demand, the electrical power from the mainfacility, not illustrated, is used to drive the alternator/motor 17 inthe manner of a motor. Under such conditions, the clutch 23 isdisengaged and the clutch 18 is engaged so that the alternator/motor l7drives the compressor 19. Air is drawn into the compressor 19 throughthe air intake 27 and is passed through a movable valve 28 which couplesthe outlet duct 29 of the compressor to a duct 31. A releasable valve 32is provided in the duct 31 and compressed air passes from the compressor19 through the valve 32 into a regenerator 33. The function andconstruction of the regenerator will be explained in more detailsubsequently in the specification.

After passing through the regenerator 33, the compressed air passesthrough a duct 34 into the storage chamber or reservoir 12. A suitablehydrostatic pressure is maintained in the chamber 12, as is known in theart, through use of the water reservoir 16 at the surface in order toprovide suitable back pressure for the stored compressed air.

During periods of high demand, the alternator/motor 17 is connected intothe electrical output system of the main plant, not shown, and driven asan alternator in order to provide an auxiliary power input thereto.Under such conditions, the clutch 18 is disengaged and the clutch 23 isengaged such that the alternator/motor 17 is coupled to be driven by theturbine 26.

In order to operate the turbine 26, the valve 28 is moved from thecondition illustrated to a condition wherein the duct 31 communicateswith the turbine inlet duct 36. The valve 32 is released so that thecompressed air stored in the reservoir 12 may pass through the duct 34,the regenerator 33, the valve 32, the duct 31, the valve 28, the duct 36and into the turbine 26. The compressed air then drives the turbine 26as it expands to atmosphere and issues through an outlet duct 37.

As previously mentioned, the air, on its way to storage in the reservoir12, passes through the regenerator 33. During compression in thecompressor 19, the temperature of the air will rise. In many systems,the work which the compressor must perform is reduced by providingintercooling stages and the volume required to store the air is reducedby providing an after-cooler stage. Such stages require separateprovision for cooling and therefore increase the amount of energyconsumed by the overall system.

In accordance with the invention, the air is compressed with noaftercooling so that it is at an elevated temperature due to the heat ofcompression when it enters the regenerator 33. The heat of compressionis thereby removed from the air and stored in the regenerator. Theregenerator effectively performs the function of an aftercooler butwithout wasting the heat. As is the case with an aftercooler, a greateramount of air may be stored for a given size reservoir since cooled airoccupies less volume. Also the necessity of utilizing power for separatecooling facilities such as intercoolers and aftercoolers may be entirelyavoided. When the air is returned to the system to drive the turbine 26,the air passes through the regenerator 33 and the heat of compression isrestored to the air, increasing the available energy for the system.

The regenerator stage 33 is illustrated as an underground cavity filledwith crushed rock. Typically, the crushed rock has a -25 percent voidfraction. Approximate dimensions for a typical system for storingsufficient thermal energy to produce 1,000,000 kilowatts of electricalpower for a 10 -hour period might employ a reservoir 12 1,400 feet belowthe surface of the reservoir 16. The reservoir 12 would have a volume ofabout 1.7 million cubic yards and the regenerator would have a volume ofapproximately 100,000 cubic yards. Economic calculations for such asystem indicate that the operating cost will be several mils perkilowatt cheaper than a system constructed in accordance with the priorart utilizing intercoolers and an aftercooler in the compression stagesand no regenerator. In addition, this system eliminates the problemsassociated with the transport and storage of fuel, and the air pollutionproblem associated with burning fuel.

The cavity comprising the regenerator should be located at a depthcapable of sustaining the expected internal pressure of the regenerator.The pressure typically will be about 600 pounds per square inch. Such apressure would require a minimum depth of about 700 to 1,000 feet.Selection of the site should consider the availability of suitable rockand it may be desirable to locate the regenerator at a deeper locationif rock more suitable for high temperature operation is available atthat elevation.

The regenerator could be constructed by preparing the cavity usingconventional hard rock mining techniques and by later filling the cavitywith crushed rock or with fabricated ceramic material. Alternatively,the cavity could be formed by mining the cavity to about 10 percent ofthe final desired regenerator volume, filling the cavity with explosive,and detonating the explosive. This would produce a pile of crushed rockwhich could be suitable for the regenerator. Conventional explosiveswould typically be adequate, but it is conceivable that some excavationcould be accomplished by the use of nuclear explosives.

In most cases, the natural rock formation in which the regeneratorcavity is formed will be suitable to seal the cavity against the loss ofpressure. In certain locations, however, it may be necessary to groutthe cavity walls to minimize leakage. In some cases, it may be desirableto provide thermal insulation surrounding the regenerator cavity, suchas the utilization of fire brick or other type of ceramic insulation.Similar thermal insulation may be desirable in the duct connecting theregenerator to the surface.

Although an underground regenerator is shown and described herein, itmay be possible to construct a regenerator located above ground level.In such a case, the regenerator would require a suitable pressure vesseldesigned to withstand the regenerator pressures, typically about 600pounds per square inch, and the regenerator temperatures, typicallyapproximately 1,200" F. Walls of the pressure vessel may be constructedto withstand high temperatures or suitable thermal insulation may beprovided between the structural portions of the walls and the interiorof the regenerator. The latter would probably be a more economicalsolution. The required volume of the regenerator, of the order of100,000 cubic yards, makes the size of the required pressure vessel forhousing the regenerator quite substantial. Accordingly, it is probablymore economical to utilize an underground cavity for the regenerator.Some circumstances may make it desirable to utilize a single undergroundcavity for both the storage reservoir 12 and the regenerator 33. If suchis desired, a concrete wall may be constructed within a single cavity toseparate the sections utilized for storage from the sections utilizedfor regeneration.

In the event that peak power requirements still exist after the supplyof air has been depleted, it may be necessary to operate the system as aconventional gas turbine, i.e., with turbine 26 driving the compressor19 and the alternator 17. In such a case, provision (not shown) is madefor adding energy in the form of heat to the air which is supplied tothe turbine by the compressor. The valve 28 may be modified to bypassthe storage chamber and regenerator.

When operating as a conventional gas turbine the alternator output isthe difference between the turbine output and the compressor power. Inorder to produce electricity during this mode of operation someprovision is required to prevent the compressor power from exceeding theturbine power. In order to accomplish this, the system of the inventionemploys a coolant source 41, such as a suitable source of pressurizedwater, together with an internal water injection system built into thecompressor 19. Water injection for the compressor 19 may be designed inaccordance with well known techniques used on jet engines, and operatesto reduce the work of compression in a manner similar to that achievableby the use of intercoolers. By providing for water injection, preferablyat several points in the compressor, the compressor power is less thanturbine power and therefore there is sufficient turbine power remainingfor the production of electricity. In this mode of operation, the outputwould probably be approximately one-third that which is achievable whenstored air is available. As a result, the system of the invention isprovided with additional flexibility in that it is capable of operationas a gas turbine system as well as a stored air system.

It may be desirable under certain circumstances to utilize either anintercooler, a heater, or both in addition to a regenerator. Such asystem is illustrated in FIG. 2. The parts of the system of FIG. 2corresponding to the parts of FIG. 1 have been given identical referencenumerals preceded by a l. The differences are that the compressor 119 isin two stages 119 and 119a and that the air is passed through anintercooler 141 between the two compressor stages. Of course, if morethan two compressors or compressor stages are used, several intercoolersmay also be desirable. In addition, a heater 142 is inser'ted in theturbine inlet duct 136-l36a.

The heater or burner 142 may be employed if additional preheating of thecompressed air prior to expansion in the turbine is desired. In such acase, the consumption of fuel is required, but the required fuelconsumption to achieve a given turbine inlet temperature is consiserablyless than a system in which a regenerator is not employed. The use of aheater together with the regenerator will increase the power output 20to 30 percent compared to a system with a regenerator but no heater.

The use of an intercooler may also be desirable under certaincircumstances. As the amount of intercooling is increased, the work ofcompression is decreased but the fuel consumption required to achieve agiven turbine inlet temperature is increased. For various combinationsof off-peak energy cost and fuel cost, the optimum power cost mayinvolve some intercooling.

It may therefore be seen that the invention provides an improved methodand system of using compressed air for storing and utilizing energy. Themethod and system of the invention provide increased efiiciency overknown prior art methods and systems, making it possible to achieve ahigher power output for a given fuel or energy consumption.

Various modifications of the invention in addition to those shown anddescribed herein will become apparent to those skilled in the art fromthe foregoing description and accompanying drawings. Such modificationsare intended to fall within the scope of the appended claims.

What is claimed is:

l. A method of using compressed air for storing and utilizing energycomprising: compressing a quantity of air and storing same, removing asubstantial amount of the heat of compression of the air and storingsuch removed heat, removing at least a part of the stored compressedair, heating the removed compressed air by restoring a substantialamount of the stored heat of compression, and deriving work from theremoved and heated compressed air by allowing it to expand.

2. A method according to claim 1 wherein no heat is removed duringcompression of the air.

3. A method according to claim 1 wherein at least a portion of the heatis removed by intercooling during compression of the air.

4. A method according to claim 1 wherein the removed compressed air isadditionally heated after restoration of the stored heat and prior toexpansion.

5. A system for using compressed air for storing and utilizing energycomprising: compressor means for compressing a quantity of air, areservoir for storing the compressed air, a regenerator between saidcompressor means and said reservoir for removing a substantial amount ofthe heat of compression of the air passing from said compressor means tosaid reservoir and for storing such removed heat, prime moving means,means for conducting at least a part of the stored compressed air upondemand from said reservoir to said prime moving means, said conductingmeans passing the removed compressed air through said regenerator forrestoring a substantial amount of the stored heat of compression to theair prior to expansion at said prime moving means.

6. A system according to claim 5 including intercooling means in saidcompressor means.

7. A system according to claim 5 including a heater for additionallyheating the compressed air between said regenerator and said primemoving means.

8. A system according to claim 5 wherein said reservoir comprises anunderground cavity, and wherein said regenerator comprises a volumefilled with crushed rock.

9. A system according to claim 8 wherein said crushed rock has a 10-25percent void fraction.

10. A system according to claim 8 wherein said regenerator comprises anunderground cavity.

1 1. A system according to claim 5 including means for cooling saidcompressor means by water injection to facilitate operation of saidsystem as a conventional gas turbine system.

1. A method of using compressed air for storing and utilizing energycomprising: compressing a quantity of air and storing same, removing asubstantial amount of the heat of compression of the air and storingsuch removed heat, removing at least a part of the stored compressedair, heating the removed compressed air by restoring a substantialamount of the stored heat of compression, and deriving work from theremoved and heated compressed air by allowing it to expand.
 2. A methodaccording to claim 1 wherein no heat is removed during compression ofthe air.
 3. A method according to claim 1 wherein at least a portion ofthe heat is removed by intercooling during compression of the air.
 4. Amethod according to claim 1 wherein the removed compressed air isadditionally heated after restoration of the stored heat and prior toexpansion.
 5. A system for using compressed air for storing andutilizing energy comprising: compressor means for compressing a quantityof air, a reservoir for storing the compressed air, a regeneratorbetween said compressor means and said reservoir for removing asubstantial amount of the heat of compression of the air passing fromsaid compressor means to said reservoir and for storing such removedheat, prime moving means, means for conducting at least a part of thestored compressed air upon demand from said reservoir to said primemoving means, said conducting means passing the removed compressed airthrough said regenerator for restoring a substantial amount of thestored heat of compression to the air prior to expansion at said primemoving means.
 6. A system according to claim 5 including intercoolingmeans in said compressor means.
 7. A system according to claim 5including a heater for additionally heating the compressed air betweensaid regenerator and said prime moving means.
 8. A system according toclaim 5 wherein said reservoir comprises an underground cavity, andwherein said regenerator comprises a volume filled with crushed rock. 9.A system according to claim 8 wherein said crushed rock has a 10-25percent void fraction.
 10. A system according to claim 8 wherein saidregenerator comprises an underground cavity.
 11. A system according toclaim 5 including means for cooling said compressor means by waterinjection to facilitate operation of said system as a conventional gasturbine system.